Internal combustion engines may include poppet valves associated with each combustion chamber and cylinder that open and close at particular times during the combustion cycle to allow a fuel-air mixture to flow into the chamber in an Otto cycle or gasoline engine, to allow air to flow into the chamber in a Diesel cycle engine, or to allow exhaust to flow out of the chamber. In common fuel rail systems, a valve lifter assembly may be configured to drive a push rod and rocker arm to control the position of the corresponding poppet valve. The valve lifter assembly may include lifter body having a pin attached to the valve body, and a roller mounted about the pin and configured to rotate about the pin. The roller may be configured to contact a cam of a cam shaft, which drives the valve lifter assembly up and down to open and close the poppet valve.
For heavy duty applications, the loads on such valve lifter assemblies may be significant, which can cause failure of one or more components of the assembly if the assembly is not constructed robustly. In some cases, the effect that certain loads have on the assembly can be amplified by stress concentrations. For example, in some valve lifter assemblies, stresses can become concentrated at the areas of engagement between the roller and the pin when the valve lifter assembly rotates about its longitudinal axis due to the engine system dynamics and the ends of the roller contact the camming surface of the cam. An amount of rotation of the valve lifter assembly is inevitable and cannot be completely removed from the system. In such cases, the rotational axes of the cam and the roller are not parallel, and the engagement causes shear stresses between the roller and the pin. In addition, the off-center contact between the roller and the cam can cause accelerated wear of their surfaces. These areas of stress concentration can act as the weakest link in an otherwise robust assembly, leading to premature failure of the interfacing components.
Some assemblies have been developed that attempt to reduce stresses in valve lifter and tappet valve assemblies. For example, U.S. Pat. No. 7,748,359, issued to Bartley et al. (“the '359 patent”), discloses a tappet assembly for a machine that may include a tappet body, a pin fixedly mounted in the tappet body, and a substantially cylindrical roller mounted about the pin. The roller may have a substantially cylindrical outer surface with a circumferential dimension and a width dimension defined by two lateral edges of the roller. The roller may be configured to provide rolling contact between the outer surface of the roller and a cam. The outer surface of the roller may be crowned such that at maximum operational loading conditions of the machine, a footprint of contact pressure from the cam is spread substantially the full width of the outer cylindrical surface of the roller. However, with such designs, the roller may contact the surface of the cam along a point or line of contact when rotation of the tappet assembly occurs, thereby creating stress concentrations in the areas of surface-to-surface contact. In addition, similar point contact and shear stress concentrations may occur at the interfacing surfaces between the roller and the pin. In view of this, a need exists for valve lifter assembly designs that can reduce the amount of rotation of the assembly, and reduce the stresses and wear between mating parts when the valve lifter assembly rotation occurs.